A laser missile system is a type of weapons guiding system whereby missiles are guided towards a target by means of a laser beam positioned on the target. U.S. Pat. No. 4,709,875 issued to Cremosnik et al., entitled “Apparatus for guiding a missile” is one of many US patents disclosing systems and methods for laser missile systems. In general, the laser beam is usually spatially encoded, and rasters, or moves in a predefined way, over a specified area at a high modulation frequency. This specified area is used to guide the missile towards the target. The spatial encoding can be achieved by altering the modulation frequency in various spatial sections of the specified area. The spatial encoding can also be achieved by altering the amplitude and wavelength in various spatial sections of the specified area, or by using phase modulation, on-off keying, frequency-shift keying and phase-shift keying, all known in the art. The modulation frequency is the frequency at which laser pulses are emitted from a laser source, and not the wavelength of the laser beam, which remains constant. In general, the modulation frequencies used fall within a definable range.
In general, a user launches a missile and positions the aiming cross of the laser beam on the target. The back end of the launched missile has a detector which continuously determines a particular characteristic of the laser beam hitting its back end. The detector provides the determined particular characteristic to a processor which determines where the launched missile is located in relation to the target. The processor then provides instructions to the on-board flight computer of the launched missile to change its flight direction, if necessary, such that it heads towards the target. As a user continuously moves the laser beam, the launched missile continuously follows the laser beam, always trying to position itself in a particular spatial section of the laser beam.
Laser missile systems are difficult to detect because very low power laser beams are used to guide the missile towards a target. In general, the intensity level of the laser beams used is only slightly higher than the intensity level of noise detected by a detector. Such low levels of intensity make it difficult to differentiate between detected laser beams and detected noise. Furthermore, laser missile systems are even more difficult to detect during the initial seconds after a laser guided missile is launched towards a target because the angle of the spread of the laser beam (herein the spread angle) is initially very large. As the missile heads towards the target, the spread angle decreases and essentially zooms in and focuses in on the target. During the initial seconds when the spread angle is very large, it is difficult to detect and identify the laser beam as originating from a laser guided missile system because the ratio of the intensity of the laser beam to the area over which it is spread is large. This ratio reduces the detected intensity of the already very low power laser beam, thereby giving the laser beam an even closer resemblance to a noise signal.
Very low power laser beam detection and angle of arrival determination systems are known in the art. U.S. Pat. No. 5,771,092 issued to Dubois et al., entitled “Wavelength agile receiver with noise neutralization and angular localization capabilities (WARNALOC)” is directed to an opto-electronic device capable of detecting the presence of a collimated beam of very low power radiation and determining its angle of arrival and wavelength. The device includes a linearly variable optical filter which allows for the transmission of a spectrum of wavelengths over its surface, superimposed over an elongated detector having at least one radiation detector element in each quadrant of the elongated detector.
Radiation incident on the filter will project two separate images of two portions of the filter onto two adjacent detector elements in separate quadrants of the elongated detector. The position of the projected images can be used to determine the wavelength of the radiation. One image is projected onto one side of the elongated detector, while the second image is projected onto the other side of the elongated detector. Each projected image on the elongated detector generates a signal. The signal generated by the second image is subtracted from the signal generated by the first image to reduce background noise. Processing electronics use the two difference signals to determine the angle of arrival of the beam of radiation.